The Chinese method is simple and produces lots of debris in orbit. If this is done a few times orbits will become unusable because the danger of collisions becomes too high. The USA can not use this method since it will block itself with it, but for China it is ok, especially as a counter-threat.
Generation in vacuum though seems to be shown only in models until now:
Seems that the reaction rate is much lower, so maybe this is not a limiting factor for building a laser.
Normally high intensities are achieved by building a pulsed laser. This produces a beam of laser pulses, which is then focussed into a tiny spot. Intensities in this spot can be alot higher than inside the laser cavity. You could achieve higher laser intensities just by building a larger laser (like http://en.wikipedia.org/wiki/N... ).
Inside the laser cavity intensities are normally limited by the effects of nonlinear optics ( http://en.wikipedia.org/wiki/N... ), which occur in all kinds of matter.
Though both the high magnetic field strengths necessary and the energy loss due to thermal radiation probably make this impractical or impossible to build.
What bollocks. I think the actual question to ask is how it's possible to create the conditions for an very large (the size of the mine)and extremely low density (the concentration of natural ore) nuclear reactor.
In the days the preference for civilian reactors was to develop further along the design of the compact high density submarine reactors. The nuclear industry never got over that. There are prototypes of large reactors with much lower power density. It's a natural question to ask how low enrichment and low density one can go.
2 billion years ago the concentration of U-235 was still 3% of the uranium. It decreased due to the shorter half-lifes of U-235.
A pressurized heave water reactor runs with today's unenriched uranium, so we are better than that already.
Though over many cell divisions these inclusions should get diluted. So I guess this is only an issue for cells that do not grow and divide any more.